That "knockout game" theory comes from Jack Cashill, who hasn't exactly been the most reliable source for truth. It doesn't fit well with the circumstances of the shooting and Cashill had to stretch things to come up with his theory.
Yes, but remember Rachel Jenteal's interview with Piers Morgan. She said Trayvon wasn't going to kill Zimmerman; he was "just gonna do a little whoopass on him". The implication was that if GZ had merely submitted to Trayvon's "whoopass", everything would have been fine. So, granted, it may not have been "knockout game", but it was "whoopass time".
That interview did not get a lot of play, and Jenteal pretty much was never seen after that.